1. Field of the Invention
This invention relates generally to solid state lighting devices and related components, systems and methods. More particularly, it relates to methods of light mixing and control from a group of semiconductor light emitters.
2. Description of the Prior Art
Incandescent light bulbs are energy inefficient. About ninety percent (90%) of the electricity consumed is converted into heat instead of light. Fluorescent light bulbs are about ten (10) times more efficient than incandescent light bulbs and solid state semiconductor emitter devices such as light emitting diodes (LEDs) are about twice as efficient as fluorescent light bulbs.
Incandescent light bulbs have a lifetime of about 750-1000 hours. Fluorescent bulbs have lifetimes between 10,000-20,000 hours but they contain mercury and are therefore not an environment friendly light source. They also exhibit less favorable color reproduction. Light emitting diodes have lifetimes between 50,000-75,000 hours, provide very good color reproduction, and are environmentally friendly.
A semiconductor light emitting device using a blue light emitting diode has a main emission peak in blue wavelength ranging from 400 nm to 490 nm. The device includes a luminescent layer containing an inorganic phosphor that absorbs blue light emitted by the blue LED and produces an exciting light having an emission peak in a visible wavelength range from green to yellow (in the range of about 530 nm to 580 nm) with a broad spectrum bandwidth.
Almost all known light emitting semiconductor devices that use blue LEDs and phosphors in combination to obtain color-mixed emission from the LEDs and excitation light from the phosphors use YAG-based or silicate-based luminescent layer as phosphors. Those solid state lighting devices have typically white color temperature of about 5000K-8500K with a low color rending index Ra of about 60-75. Such a white solid state lighting device is not desirable for some applications such as indoor applications that require warm white color at about 2700 k-3500K with a high color rending index Ra above 80.
Another issue faced by conventional solid state lighting devices is the need to further improve luminous efficacy to produce higher energy efficiency with less thermal dissipation so that they can better compete with fluorescent bulbs in high volume and cost effective commercial and residential applications.
To provide a warm white light, warm white semiconductor light emitting solutions use a blue LED with a mixture of YAG-based or silicate-based phosphors for exciting yellow light and nitrides or sulfides phosphors for exciting red light. YAG-based or silicate-based phosphors excite broad-band yellow light, but have a shortage in red light and bluish green light, which limits their color rendering index Ra to less than 70. Adding red phosphor to yellow phosphor can compensate for a shortage of red light, resulting in improved color rendering index of about 75-80. However, red phosphor absorbs blue light with peak wavelength around 460 nm and excites red light with peak wavelength around 620 nm, which causes a significant Stoke-shift issue in photonic energy loss.
A new method for rendering warm white semiconductor light emitting devices was proposed recently (2008) by using a blue LED with YAG-based or silicate-based phosphors for exciting yellow light or blue shifting yellow light and mixing that light with semiconductor emitting red/amber color light. Adding light from a red/amber semiconductor emitter directly to light from a solid state white lighting device solves multi-phosphors self-absorption loss and Stokes shift loss of blue-to-red wavelength conversion. Efforts are ongoing to improve the light mixture from multi-color semiconductor light emitters.
There remains a need, therefore, for multi-color semiconductor light emitters having improved light mixtures.
More specifically, there is a need for an effective color mixing solution for multi-spectrum semiconductor emitters in a warm white solid state lighting device.
However, in view of the prior art taken as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the identified need could be fulfilled.